An OWL Ontology of Norms and Normative Judgements

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1 An OWL Ontology of Norms and Normative Judgements Rossella Rubino, Antonino Rotolo, Giovanni Sartor CIRSFID, Alma Mater Studiorum - Università di Bologna Via Galliera, 3, I Bologna, Italy Law Department, European University Institute Via Boccaccio, 121 I Firenze,Italy Abstract. In this paper we present an OWL ontology of fundamental legal concepts developed within the ESTRELLA European project. The ontology includes the basic normative components of legal knowledge: deontic modalities, obligative rights, permissive rights, liberty rights, liability rights, different kinds of legal powers, potestative rights (rights to produce legal results) and sources of law. Besides the taxonomy the ontology comprises also the semantic relations between the concepts. The aim of the paper is that the proposed ontology may be useful for semantic access to digital legal information and for the representation of legal knowledge. Keywords: ontology, OWL, legal concepts Introduction In this paper we shall provide an OWL ontology of fundamental legal concepts introduced in (Sartor, 2006) with the twofold aim of clarifying the basic normative components of legal knowledge and of contributing to enable semantic access to digital legal information. The ontology has been developed under the European project ES- TRELLA 1 which aims to develop and validate an open, standards-based platform allowing public administrations to develop and deploy comprehensive legal knowledge management solutions, without becoming dependent on proprietary products of particular vendors. The main technical objectives of the ESTRELLA project are to develop a Legal Knowledge Interchange Format (LKIF), building upon emerging XMLbased standards of the Semantic Web, including RDF and OWL, and Application Programmer Interfaces (APIs) for interacting with LKIF legal knowledge systems. The formal language chosen to express this first version of our ontology is OWL-DL (Smith et al, 2004) 2, which is an OWL version directly translatable into Description Logic and for which there are 1 ESTRELLA - The European project for Standardized Transparent Representations in order to Extend Legal Accessibility (IST ) 2

2 174 Rubino, Rotolo, Sartor several inference engines. The ontology has been developed using the OWL Plugin of the Protégé ontology editing tool. According to (Sartor, 2006), the set of fundamental legal concepts includes, besides the usual deontic modalities (obligation, prohibition and permission), other notions like the following: obligative rights (rights related to others obligations), permissive rights, erga-omnes rights, liberty rights, liability rights, different kinds of legal powers, potestative rights (rights to produce legal results) and sources of law. Of course, we also need some more general concepts such as those of an agent and an action which an agent can perform, or again, the concept of a situation which an agent can create. In this paper we assume that these concepts can be imported from top and core ontologies such as DOLCE 3 ontology, LRI-core 4 ontology and so on. This paper is structured as follows. In Section 1 we give a taxonomy of the fundamental legal concepts introduced in (Sartor, 2005; Sartor, 2006) along with their definition. In Section 2 we describe in more detail the structure of the ontology in terms of classes and properties. For space reasons full OWL code is not provided here but it can be requested to the authors. Finally, conclusions and future work are drawn in Section A Taxonomy of Legal Concepts A first classification of legal concepts, as depicted in Figure 1, includes two main classes: Norms and Normative judgements. In the following Figure 1. Relationships among main legal concepts subsections we will analyze these macro-concepts and how they can be further classified Normative Judgements A normative judgement is the propositional constituent expressing a normative fact and can be classified (see Figure 2) into the following kinds:

3 An OWL Ontology of Norms and Normative Judgements 175 evaluative, which indicates that something is good or bad, is a value to be optimised or an evil to be minimised (for example, human dignity is value, participation ought to be promoted ); qualificatory, which ascribes a legal quality to a person or an object (for example, x is a citizen, x is an intellectual work, x is a technical invention ); definitional, which specifies the meaning of a term (for example x means y or by x it is meant y ); deontic, which imposes the obligation or confers the permission to do a certain action (for example x has the obligation or x has the permission to do A ); potestative, which attributes powers (for example a worker has the power to terminate his work contract ); evidentiary, which establishes the conclusion to be drawn from certain evidence (for example it is presumed that dismissal was discriminatory ); existential, which indicates the beginning or the termination of the existence of a legal entity (for example the company ceases to exist ); norm-concerning judgements, which state the modifications of norms such as abrogation, repeal, substitution, and so on. In the following we will analyze in more detail the concepts of deontic judgements and potestative judgements Deontic judgements A deontic judgement expresses the fact that a certain content is qualified by deontic modalities, such as typically obligation, prohibition and permission. Deontic modalities: permissive judgements and obligative judgements (see Figure 3). Deontic concepts can be reduced to those of obligation and permission. Classically, prohibition is defined in terms of the notion of obligation (OBL ). We shall not consider here the distinction between weak (or negative) permission, consisting in the non-derivability of a prohibition, and strong permission, equivalent to the explicit negation of a prohibition. Basic deontic modalities (Basic Obligative and Basic Permissive) correspond the standard deontic qualifications in deontic discourse.

4 176 Rubino, Rotolo, Sartor Figure 2. A taxonomy of normative judgements Figure 3. A taxonomy of deontic normative judgements They are also called undirected deontic modalities, as no explicit reference is made to any subject which may be the beneficiary of the deontic qualification. Basic deontic modalities can be further classified into ought-to-be and ought-to-do judgements: the former express deontic qualifications whose content are states of affairs without necessarily mentioning actors or actions bearing relations with such states of affairs; the latter may be interpreted as expressing deontic qualifications of explicit actions. Although in many cases ought-to-be statements can be reframed as ought-to-do statements, it is quite controversial that this can be done

5 An OWL Ontology of Norms and Normative Judgements 177 in general and ought-to-be statements are often made when it is not known who will have the responsibility of realising the state of affairs though it is known that somebody has this responsibility. An example of normative judgement involving an undirected oughtto-be qualification is the following: The balance of a bank account ought to be non-negative. An example of normative judgement involving an undirected oughtto-do qualification is the following: Everybody has the obligation to pay taxes. Normative judgements stating directed deontic modalities, that is Directed Obligative and Directed Permissive, indicate the beneficiaries of the deontic qualifications specified in such judgements. We distinguish two ways in which such an indication can take place: either the deontic qualification holds towards specified individuals, in which case we speak of an individualised qualification, or it holds towards everybody, in which case we speak of an erga-omnes qualification. An example of normative judgement involving a directed erga-omnes ought-to-be qualification is the following: In the interest of the every Italian citizen traffic ought to be reduced. An example of normative judgement involving a directed individualised ought-to-do qualification is the following: In the interest of Mr. Jones, Ms. Smith has the obligation to pay him one thousand euros. An example of normative judgement involving a directed erga-omnes ought-to-do qualification is the following: In the interest of the owner everybody is forbidden to use his/her property without his/her consent. We represent the distinction between individualised deontic qualification and erga-omnes deontic qualification through the values assigned to the property towards, which will concern individually named agent in the case of individualised judgements, and the all agents in the case of erga-omnes judgements. We will see the properties of each class in the next section. Directed obligative ought-to-do are also called obligative rights. k has the obligative right that j does A iff it is obligatory, towards k, that j does A. An example of obligative right is it is obligatory, towards Mary, that Tom pays 1,000 euros to John. Another type of obligative rights are the exclusionary rights which concern the prohibition against performing certain inferences (against reasoning in certain ways), or against using certain kinds of premises for certain purposes, in the interest of a particular person. This is especially the case with anti-discrimination rules. For instance, in many legal systems employers are prohibited from adopting any decision having a negative impact on their employees on the basis of race or sex, and this prohibition, though also serving some collective purposes, is primarily

6 178 Rubino, Rotolo, Sartor aimed at promoting the interest of the employees in question. Let us now specifically consider how we can conceptualise the difference between directed ought-to-do deontic judgements having a positive or a negative content (see Figure 4), that is, concerning actions or omissions. Both obligations and permissions can be divided into positive and negative according to whether they concern an action or an omission. Figure 4. A taxonomy of positive and negative judgements Directed negative permissions constitute what is also called privilege in the Hohfeldian language (Hohfeld, 1913; Hohfeld, 1917): j has a privilege towards k, with regard to action A, iff it is permitted towards k that j omits to do A. Always following Hohfeld, we may use the less controversial expression noright to express that one does not have the obligational right that another does a certain action, that is, to denote the situation when the latter is permitted towards the former to omit that action. Therefore, we can say that k has a noright that j does A iff j is permitted, towards k, to omit A. Let us make an example both for privileges and norights. Assume for instance that Mary, a writer, has made a contract with Tom, a publisher, and has committed herself to write a novel for him. Mary s privilege would consist in the Mary having permission towards Tom not to write the novel, a normative situation which could also be described as Tom s noright that Mary writes the novel.

7 An OWL Ontology of Norms and Normative Judgements 179 Positive and negative permissions, as showed in Figure 5, are merged into the concept of faculty (for instance, by saying that a woman has the faculty of wearing a miniskirt when going to work, we mean that it is permitted both to wear it and not to wear it). Figure 5. A taxonomy of faculties When, for the benefit of a person, this person is both permitted to perform and to omit an action that is, when the action is facultative we can say that he or she has a liberty right with regard to that action. This notion can be further developed according to the fact that others (or the government) may have, always in the interest of that person, a prohibition to prevent the facultative action, and they may even have the obligation to provide means for its performance. This leads us to distinguish three kinds of liberty rights: a mere liberty right, a negatively protected liberty right, and a positively protected liberty right. In general we speak of a right to characterise the situation where a normative judgement is intended to benefit a particular person. According to this notion of a right, the directed obligations of agent j for the benefit of agent k can be viewed as k s right, namely as k s obligative right towards j. The negation of a directed obligation is a directed permission. However, it counts as a right, namely, a permissive right, only when such negation is aimed at benefitting the author of the permitted action. Another notion of a right is that of liability rights. That j has a liability right concerning k s action A means that if k performs the permitted action A then k will have to perform another action B for the benefit of j. For example, consider a copyright regime when one is permitted to reproduce a protected work, but the author is entitled to a royalty for the reproduction of his or her work. In this case we have a normative connection between a permitted action and an obligation of the agent, to the benefit of another.

8 180 Rubino, Rotolo, Sartor However for us this kind of legal position represents a conditional, namely, a norm, rather than a normative judgement Potestative judgements Potestative judgements concern the attribution of powers. The first level of our classification (see Figure 6) includes the categories Hohfeldian powers, enabling powers and declarative powers: the first covers any action which determines a legal effect, the second only cases when the law aims at enabling the agent to produce the effects in this way, the third the case when the effect is produced through the party s declaration of it. Figure 6. A taxonomy of potestative judgements In more detail we say that j has the declarative power to realize A to mean that if j declares A, then it is legally valid that A. For example, if x has the declarative power to terminate y s obligation towards x to do then if x declares that y s obligation towards x finishes, then it is legally valid that this obligation finishes. The second Hohfeldian square includes immunities, action powers, subjections (the normative position that Hohfeld denotes as liability), and disabilities. Immunities. k has an immunity towards j with regard to the creation of position Pos in the head of k, exactly if is not the case that j has that power. Action powers. An action-power consists in a generic power to produce a legal effect through an action determining it. Subjections. That k is in a state of subjection towards j, with regard to normative position Pos, means that j has the abstract enabling-power of determining Pos in the head of k. For instance, debtor k is subject to creditor j in relation to j s power of freeing k from j s obligation. Disabilities. j has a disability towards k, with regard to the creation of position Pos exactly if it is not the case that j has the abstract enabling power of creating Pos in the head of x. A special kind of enabling powers, called potestative rights, can be distinguished, that is powers which are meant to benefit the holder of

9 An OWL Ontology of Norms and Normative Judgements 181 the power. For example, if y does not belong to anybody, then x has the potestative-right to start x s ownership of the animal, by capturing y Other possible classifications Our model does not exhaust all possible classifications of norms and of their components. For instance we could consider the antecedents of conditioned norms, and introduce the traditional classification between juridical fact, acts (facts relevantly determined by humans), and declarations of will or intentions. In this way we might also characterise the notion of a source of law, by which we mean any fact that embeds normative propositions and makes them legally valid by virtue of such an embedment. Some sources of the law are events (like the issuing of a high court decision), while others are state of affairs (like the practice of a custom or a result declaration) Norms Norms are propositions stating normative judgements. Norms can be unconditioned, that is their judgement may not depend upon any antecedent condition (consider, for example. the norm everyone has the right to express his or her opinion ). Usually, however, norms are conditioned. We distinguish conditioned norms into rules, which make a normative judgement dependent upon defeasibly sufficient conditions or factor links, which make a normative judgement dependent upon contributory conditions. Figure 7. A taxonomy of norms As showed in Figure 7 rules can be distinguished in: initiation rules, that is, rules stating that a certain normative proposition starts to hold when the rule s conditions are satisfied. An example is if one causes a damage, one has to compensate it ;

10 182 Rubino, Rotolo, Sartor termination rules, that is, rules stating that a normative proposition ceases to hold when the rule s conditions are satisfied. An example is if one pays a debt, the obligation terminates ; supervenience rules, that is, rules stating that a normative proposition holds as long as the conditions are satisfied. An example is if one is in a public office, one is forbidden to smoke. As we said factor-links make a normative judgement dependent upon contributory conditions: in this case the condition favours the judgement, but it does not determine it, not even defeasibly. It seems to us that factor links usually are of the supervenience kind, and thus we will include only this option in our ontology. For example if a work has educational values, this favours the conclusion that it is covered by free use is a supervenience factor link since the conclusion is favoured while the condition (the work has educational values) holds. 2. The Ontology Structure Besides taxonomy, an OWL ontology usually provides a description of concepts in terms of classes and properties (Noy and McGuinness, 2001). The word concept is sometimes used in place of class. Classes are a concrete representation of concepts. Therefore, all the concepts examined in the previous section constitute classes organized into a superclass-subclass hierarchy (the taxonomy) already described. Subclasses specialise ( are subsumed by ) their superclasses. For example, as depicted in Figure 1, the Norms class and the Normative judgements class are subclasses of the Thing class that is the class which contains everything (everything is a thing). Features or attributes of a class are called its properties and normally, because of inheritance, properties are defined for the highest class in the hierarchy to which they apply. The domain of a property is the class of objects the property can be applied to, while the range defines the values the property can take. In OWL, a clear distinction is made between two types of properties (Antoniou and van Harmelen, 2004): owl:objectproperty, which relate objects to other objects, and owl:datatypeproperty, which relate objects to datatype values. The conditioned_by property is an owl:datatypeproperty because the domain is the Conditioned norms class and the range is the xsd:string class. All other properties are instead owl:objectproperty and they are: the state property;

11 An OWL Ontology of Norms and Normative Judgements 183 Figure 8. The conditioned_by property the exercised_through property; the power_holder property; the concerned property; the bearer property; the towards property. The state property is derived from the definition of norm that is Norms state normative judgements. Indeed, the Norms class is the domain and the Normative judgements class is the range. Figure 9. The state property The exercised_through property describes the action through which the power is exercised. The power_holder is the agent who performs this action. While the Potestative judgements class is the domain of both the power_holder and the exercised_through property, the range is different. Indeed, the power_holder property has the Agents class as range and exercised_through property has the Actions 5 class as range. Figure 10. The exercised_through property 5 We assume that the Agents and the Actions class are imported from some top ontology.

12 184 Rubino, Rotolo, Sartor Figure 11. The power_holder property Similarly to the potestative judgements, we defined two properties for the deontic modalities concerning actions: the concerned and the bearer properties. The concerned property describes the action permitted or obligatory which is performed by an agent called bearer. The domain of the concerned property and the bearer property is the union of four classes: the Basic Obligative ToDo class, the Directed Obligative ToDo class, the Basic Permissive ToDo class and the Directed Permissive ToDo class. The range is the Agents class for the bearer property and the Actions class for the concerned property. Figure 12. The concerned property Finally, the towards property describes the beneficiary of directed deontic normative judgement. The domain, in this case, is represented by the Directed Obligative class and the Directed Permissive class while the range is represented by the Agents class. This property also enables to represent the distinction between individualised deontic qualification and erga-omnes deontic qualification through the values assigned to the property towards, which will concern individually named agent in the case of individualised judgements, and the all agents in the case of erga-omnes judgements. For each property can be defined relational characteristics such as functionality, inverse functionality, symmetry and transitivity. If some

13 An OWL Ontology of Norms and Normative Judgements 185 Figure 13. The bearer property Figure 14. The towards property property links value a to value b then its inverse property will link value b to value a. If a property is inverse functional then it means that the inverse property is functional. Functional properties are also known as single valued properties and also features. If a property P is transitive, and the property relates value a to value b, and also value b to value c, then we can infer that value a is related to value c via property P. If a property P is symmetric, and the property relates value a to value b then value b is also related to value a via property P. The exercised_through property and the concerned property are functional because the action involved is only one. The power_holder property, the bearer property and the towards property are functional because the agent which perform the action or the beneficiary are single agents. Properties may be enriched with other facets which will be considered in future work.

14 186 Rubino, Rotolo, Sartor 3. Conclusion and Future work The ontology presented in this paper represents the first attempt of classifying such type of legal concepts. In the literature, ontologies such as LRI-core examine top concepts such as action, intention, social role, legal action, normative article, and so on. Other ontologies are instead too specific since they consider specific legal domains such as, for example, the intellectual property rights (IPROnto (Delgado et al, 2003)). In future work we intend to extend the ontology by importing core and top ontologies for main legal concepts and for all the modifications that a norm may have in the time (norms concerning judgements). Moreover, class properties will be specified in more detail as regards disjointedness or equivalence among classes, property cardinality, property restrictions and default values for properties. Finally, the problems of the expressive power of OWL in representing concepts usually described by using logic formalisms will be analyzed. Acknowledgements The work reported here has been partially financed also by the EU projects IST ALIS, MEXC ONE-LEX. References Antoniou, G. and van Harmelen, F. A Semantic Web Primer MIT Press, Delgado, J., Gallego, I., Llorente, S. and Garcia, R. IPROnto: An Ontology for Digital Rights Management. 16th Annual Conference on Legal Knowledge and Information Systems, JURIX Frontiers in Artificial Intelligence and Applications, Vol. 106, IOS Press, Hohfeld, W. N Some Fundamental Legal Conceptions as Applied in Judicial Reasoning. I. Yale Law Journal 23: Hohfeld, W. N Some Fundamental Legal Conceptions as Applied in Judicial Reasoning. II. Yale Law Journal 26: Noy, N. F. and McGuinness, D. L. Ontology Development 101: A Guide to Creating Your First Ontology. Stanford Knowledge Systems Laboratory Technical Report KSL and Stanford Medical Informatics Technical Report SMI , March Sartor, G. Legal Reasoning: A Cognitive Approach to the Law. Springer, Part. 1. Legal Reasoning and Legal Rationality, pp : Part. II. Legal Logic, pp Bibliography and references: pp Sartor, G. Fundamental Legal Concepts: A Teleological Characterisation. Artificial Intelligence and Law, to be published.

15 An OWL Ontology of Norms and Normative Judgements 187 Smith, M. K., Welty, C. and McGuinness, D. L. OWL Web Ontology Language Guide. W3C Recommendation 10 February